State-by-state marijuana legalization has existed on the sufferance of the Democratic party on this issue. There is an immense amount of power for the Federal government to crack down on the this if they choose, and Donald Trump has never shied away from engaging in exactly such culture-war issues.
I would not even be surprised to see medical usage rolled back. There's plenty of justification: after all, Schedule I drugs are supposed to have "no legitimate medical uses", despite the reality of the drugs on it. But it will largely come down to culture war: medical marijuana is exactly the camel's nose under the tent that conservatives said it would be.
Politically, legalization is a rallying call for Democrats that pushes turnout. It can be easily snuffed by Republicans, and it will be - the same way the drug war was used in the 70s and 80s.
America made some bad choices last night and we're about to get a nice show of just how much power the Federal government can wield when one party both completely controls all the branches of government and has the will to use it as a blunt instrument of power. States' rights are just a platitude for when Democrats are in power, it's not going to slow Republicans down in the least.